As he left court on unconditional bail, Huhne made a brief statement, saying said he was resigning his parliamentary seat after taking responsibility for something that happened 10 years ago.

On the first day of what was to be his trial at Southwark crown court, the Liberal Democrat MP entered a plea of guilty over claims his ex-wife took speeding points for him 10 years ago.

Huhne, 58, the former energy secretary, had pleaded not guilty to the charge at a hearing last week.

The charges relate to allegations that in 2003 he persuaded his then wife, Vicky Pryce, 60, an economist, to take speeding points so that he could avoid prosecution.

Carina Trimingham, 46, the PR adviser for whom Huhne left his wife in 2010 after 26 years of marriage, sat in the public gallery behind the dock as the charge was again put to him on Monday morning.

Standing in the dock, Huhne, MP for Eastleigh in Hampshire, spoke clearly as he said "guilty" when asked how he pleaded.

He was charged with doing an act tending or intending to pervert the course of justice between 12 March 2003 and 21 May 2003.

Pryce has pleaded not guilty to the charge on the grounds of marital coercion.

Her case was adjourned until 10am on Tuesday at Southwark, when a trial is expected to start. She was granted unconditional bail.

The charges relate to an incident in March 2003 when Huhne's car was allegedly caught by a speed camera on the motorway between Stansted airport and London. The allegation emerged in 2011 after the end of his marriage.

Huhne was granted unconditional bail and will be sentenced at a later date.

Huhne stepped down from the cabinet in February last year after he was charged. At the time he vowed he would fight the charges.

Granting Huhne unconditional bail, judge Mr Justice Sweeney said: "I will deal with your sentence on a date to be notified. It is essential that you attend, on that date."

He added that as Huhne's barrister, John Kelsey-Fry, had foreshadowed: "You should have no illusion whatsoever as to the sort of sentence that you are likely to receive."

Huhne, whose father was also in the public gallery, indicated he understood.

As he left the court. Huhne said: "I pleaded guilty today. I am unable to say more while there is an outstanding trial. But having taken responsibility for something which happened 10 years ago the only proper course of action is now for me to resign my Eastleigh seat in parliament, which I will now do very shortly."